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Aeolian transport of mineral dust is an important carrier of nutrients and trace metals to the surface ocean and, hence, affects the biogeochemical cycles of C and N therein. Studies focusing on the provenances of dust across the latitudinal gradients over the open ocean waters are essential for better constraining the biogeochemical effects of dust deposition. Here, we analyzed the radiogenic isotope composition of Sr (87Sr/86Sr) and Nd (ε Nd (0)) in the silicate fraction of mineral aerosols collected over the pelagic northern and southern Indian ocean waters during the GEOTRACES-India research expeditions (GI-01: March-April 2014 and GI-02: April-May 2014). Both tracers, 87Sr/86Sr and ε Nd (0), exhibited marked spatial variability for dust deposition to different basins, including the Bay of Bengal (BoB), Arabian Sea (AS), equatorial Indian Ocean (EIO), and the southern sector of the Indian Ocean (SIO). The dust deposited in the AS showed a narrow spread in both 87Sr/86Sr (0.71520–0.71642) and ε Nd (0) (−10.9 to −10.1), along with the air mass back trajectories, indicating transport from the Arabian Peninsula. Likewise, the dust collected over the BoB was mainly sourced from the Thar Desert (87Sr/86Sr > 0.72, −16 < ε Nd (0) 〈 −1 3), whereas the EIO received mineral dust from the Thar Desert and sand dunes in northern Australia (87Sr/86Sr: 0.72, ε Nd (0): −11) and Indonesian Archipelago. In contrast, the SIO appeared to receive dust deposition from three different source regions, including dust from Antarctica, western Australia (87Sr/86Sr: 0.712, −20 < ε Nd (0) <−16, and South Africa/South America (87Sr/86Sr: 0.708–0.710; −24.5 < ε Nd (0) >−8.4). These results have important implications for modelling the dust transport to pelagic Indian and southern Indian Ocean waters. [ABSTRACT FROM AUTHOR]

The variability of Indian Ocean shallow meridional overturning circulation (SMOC) is studied using the century long ocean reanalysis simple ocean data assimilation (SODA) data. Though SMOC exhibits stronger southward transport during boreal summer, it displays stronger variability during boreal winter. The spectrum analysis of winter SMOC index reveals presence of highest amplitude between 5 to 7 years at 95% confidence level, suggesting the dominance of intra-decadal SMOC variability. The robustness of intra-decadal SMOC variability is also confirmed in different ocean reanalysis data sets. Composite analysis of filtered upper Ocean Heat Content, sea level, thermocline depth and Sea Surface Temperature anomalies for strong (weak) SMOC years show negative (positive) anomaly over north and East of Madagascar. Correlation analysis, of filtered SMOC index and sea level pressure (zonal winds) over the India Ocean, found significant negative (positive) correlation coefficient north of 40 °S (around 10 °S) and significant positive (negative) correlation coefficient over the 45 °S to 70 °S (20 °S to 50 °S and north of 5 °S). This meridional pattern of correlation coefficient for sea level pressure, manifesting the out of phase relationship between sub-tropics and high latitude mean sea level pressure, resembles with Southern Annular Mode (SAM). We conclude that the intra-decadal variability of mean sea level pressure leads to zonal wind variation around 10 °S modulating SMOC, which in turn affects the upper ocean thermal properties in the east and north of Madagascar. This study for the first time brought out coherent intra-decadal evolution of SAM and SMOC during boreal winter. [ABSTRACT FROM AUTHOR]

Monitoring the presence of blue whale (Balaenoptera musculus ssp.) stereotyped calls has been a widely used method to assess the different populations' distribution worldwide. All blue whale populations also produce nonstereotyped vocalizations, or D‐calls. Here, we monitored the presence of D‐calls in long‐term records from a large hydrophone array located in the open southern Indian Ocean, using an automated detection method and manual validation of the detections. D‐calls were detected at all sites of the array, which extends from 24°S to 56°S, but the majority of them were detected at the two southernmost sites. We observed a latitudinal shift in their seasonal occurrence, with more D‐calls in the north during austral autumn and winter and more in the south during austral spring. The geographical occurrence of D‐calls compared to that of songs indicates that blue whale acoustic behavior switches from a song‐intensive and sparse‐D‐call emission in the north to song‐moderate and more intensive D‐call emissions in the south. These findings support the hypothesis that both call types are used for different purposes, as D‐calls are mainly detected around foraging grounds and songs in wintering grounds. Monitoring both call types might therefore be a relevant acoustic indicator of blue whale behavior. [ABSTRACT FROM AUTHOR]

Polychlorinated biphenyl compounds (PCBs) are industrial chemicals whose production was discontinued in the early nineties in most countries. Sill, PCBs are detectable in pristine and remote locations. Occurrence in regions such as Southern Oceans and Antarctica are influenced by the global, and regional, cycling. Here, we studied the surface and deep ocean distribution of indicator- and dioxin-like PCB congeners in the Southern Indian Ocean (SIO), and the coast of Antarctica (COA) during the tenth Indian Southern Ocean Expedition (SOE-10), December 2017–February 2018. ∑ 21 PCBs in SIO surface waters ranged from 3.8 to 167.1 pg L−1 (average ± standard deviation: 35.7 ± 48.4 pg L−1), and in COA from 1.0 to 41.8 pg L−1 (13.8 ± 12.7 pg L−1), respectively. A noticeable gradient was observed, with higher PCBs levels in northern latitudes than southern latitudes in the SIO, and higher levels in the eastern longitudes compared to western longitudes in the COA. Results suggest the influence of secondary sources, or re-emission, of PCBs in the Southern Oceans and Antarctica. Both regions showed notable PCB levels in surface and deep waters (up to 1000 m) due to ongoing surface sources and remineralization processes in deeper waters. Multimedia modeling with the global model (BETR-Global) suggests the SIO act as a net sink for PCBs in the ocean. [Display omitted] • Despite the decline, secondary PCB sources or remissions persist in both regions. • PCB presence in surface and deep waters suggests ongoing sources and mineralization. • Multimedia modelling suggests the SIO and COA are net sinks for PCBs. • Persistence of secondary sources in polar regions suggests ongoing contamination. [ABSTRACT FROM AUTHOR]

In the Southern Ocean, the term "winter waters" (WWs) refers to a water mass characterized by a subsurface layer of minimum temperature that plays an important ecological role for marine ecosystems, and in particular for top predators. Given that the Southern Ocean is experiencing warming and intense marine heatwaves (MHWs), particularly at subantarctic latitudes, we investigate here how different levels of warming might impact the presence, depth and minimum temperature of WWs in the Indian sector of the Southern Ocean. In particular, we assess how WWs are impacted by surface MHWs using in situ Argo hydrographic observations and biologging data. The results indicate that WWs are substantially reduced, deeper and warmer during the presence of MHWs. Using the most recent climate projections, we find a significant, but scenario-dependent, southward shift of WWs under global warming. Potential impacts of such WW shifts on pelagic ecosystems, at different timescales (from daily to decadal), are discussed. • Marine heatwaves are associated with a decreased probability of presence of winter waters. • Winter water distribution shifts southward linearly with global warming west of Kerguelen Plateau. • Winter water distribution southward shift is weak east of Kerguelen up to 2.6 °C of global warming. [ABSTRACT FROM AUTHOR]

Using 28-year satellite-borne Special Sensor Microwave Imager observations, features of high-wind frequency (HWF) over the southern Indian Ocean are investigated. Climatology maps show that high winds occur frequently during austral winter, located in the open ocean south of Polar Front in subpolar region, warm flank of the Subantarctic Front between 55°E–78°E, and south of Cape Agulhas, where westerly wind prevails. The strong instability of marine atmospheric boundary layer accompanied by increased sensible and latent heat fluxes on the warmer flank acts to enhance the vertical momentum mixing, thus accelerate the surface winds. Effects of sea surface temperature (SST) front can even reach the entire troposphere by deep convection. HWF also shows distinct interannual variability, which is associated with the Southern Annual Mode (SAM). During positive phase of the SAM, HWF has positive anomalies over the open ocean south of Polar Front, while has negative anomalies north of the SST front. A phase shift of HWF happened around 2001, which is likely related to the reduction of storm tracks and poleward shift of westerly winds in the Southern Hemisphere. [ABSTRACT FROM AUTHOR]

• Under all scenarios, an intensification of warming and marine heatwaves is projected between 40°S and 55°S. • Projected climate velocities are faster in the mesopelagic than at the surface. • SSP1-2.6 and SSP2-4.5 warming and MHW patterns mostly differ in the long term. The Southern Indian Ocean (20–120°E, 70–30°S) hosts an exceptional biodiversity that contributed to the inscription of the French and Australian natural reserves on the UNESCO World Heritage List. This region is a "hot spot" for ocean heat uptake and already experiences intense marine heat waves (MHW), as evidenced in 2011/2012 over the Kerguelen Plateau. In the coming decades, this region is also expected to face supplemental anthropogenic warming, depending on future greenhouse gas emissions, with unknown consequences for its marine ecosystems. Here, we present a regional analysis of ocean warming and MHW based on the analyses of historical observations and Coupled Model Intercomparison Project Phase 6 (CMIP6) climate projections. Consistent with observations over the last decades, we find an intensification through the 21st century of surface warming and MHW over a band located between 40°S and 55°S within the Antarctic Circumpolar Current region. CMIP6 models also project much faster climate velocities (i.e. the speed and direction at which isotherms drift in the wake of climate change) in the mesopelagic (200–1000 m) than at the surface (0–200 m). Lastly, a comparison between the two Shared Socioeconomic Pathways (SSP1-2.6 and SSP2-4.5) analysed in this study shows much larger changes in the second half of the 21st century for the higher emission scenario. These results suggest that the subantarctic islands will probably be mostly affected by warming and MHW under both scenarios, although committing to SSP1-2.6 could substantially alleviate the pressure on ecosystems in the long term. This study also highlights the need to consider a tri-dimensional environment that may evolve at different paces when designing efficient conservation measures. [ABSTRACT FROM AUTHOR]

The low-frequency atmosphere-ocean coupled variability of the southern Indian Ocean (SIO) was investigated using observation data over 1958-2010. These data were obtained from ECMWF for sea level pressure (SLP) and wind, from NCEP/NCAR for heat fluxes, and from the Hadley Center for SST. To obtain the coupled air-sea variability, we performed SVD analyses on SST and SLP. The primary coupled mode represents 43% of the total square covariance and is featured by weak westerly winds along 45°-30°S. This weakened subtropical anticyclone forces fluctuations in a well-known subtropical dipole structure in the SST via wind-induced processes. The SST changes in response to atmosphere forcing and is predictable with a lead-time of 1-2 months. Atmosphere-ocean coupling of this mode is strongest during the austral summer. Its principle component is characterized by mixed interannual and interdecadal fluctuations. There is a strong relationship between the first mode and Antarctic Oscillation (AAO). The AAO can influence the coupled processes in the SIO by modulating the subtropical high. The second mode, accounting for 30% of the total square covariance, represents a 25-year period interdecadal oscillation in the strength of the subtropical anticyclone that is accompanied by fluctuations of a monopole structure in the SST along the 35°-25°S band. It is caused by subsidence of the atmosphere. The present study also shows that physical processes of both local thermodynamic and ocean circulation in the SIO have a crucial role in the formation of the atmosphere-ocean covariability. [ABSTRACT FROM AUTHOR]

The dynamic importance of the Southern Indian Ocean (SIO) lies in the fact that it connects the three major world oceans: the Pacific, Atlantic, and Indian Oceans. Modeling study has been used to understand the circulation pattern of this very important region. Simulation of SIO (10°N-60°S and 30°E-120°E) is performed with z-coordinate Ocean General Circulation Model (OGCM) viz; MOM3.0 and the results have been compared with observed ship drift data. It is found that except near coastal boundaries and in equatorial region, the simulated current reproduce most well known current pattern such as Antarctic Circumpolar Current (ACC), South Equatorial Current (SEC) etc. and bears a resemblance to that of the observed data; however the magnitude of the surface current is weaker in model than the observed data, which may be due to deficiency in the forcing field and boundary condition and problem with observed data. The annual mean wind stress curl computed over the oceanic domain reveals about ACC and its similar importance. The way in which the ocean responds to the windstress and vertically integrated transport using model output is fascinating and rather good. [ABSTRACT FROM AUTHOR]

As a new type of pollutant, microplastics widely exist in the marine environment and have attracted a lot of attention from the international community. In order to study the distribution of microplastics and the influence of ocean current, microplastic samples in seawater of the southern Indian Ocean were collected using a peristaltic pump equipped on-board and concentrated on site. Qualitative and quantitative analyses of microplastics were performed using a stereo-microscope and a micro-Fourier transform infrared spectroscope attenuated total reflection. The results showed that the average abundance of microplastics in seawater of the southern Indian Ocean was 2.3 ± 2.1 items/m3, which was consistent with that in other oceans. Polyethylene terephthalate (PET), polyethylene (PE), Rayon, polyamide (PA), and polyvinylidene chloride (PVDC) were the main polymers of microplastics in the southern Indian Ocean. The size range of all detected microplastics was 108.2–4703.0 µm. All microplastics had different colors, such as black, red, yellow, gray, blue, green, purple, and transparent. Fiber was the dominant shape of microplastics. The abundance distribution of microplastics fluctuated in the latitudinal direction. The abundance of microplastics from the present study area was higher in the coastal region of the South Africa continent and the Indian Ocean garbage patch, with an average abundance of 4.0 items/m3. The average abundance of microplastics was relatively high in the convergence area of the circulation, which revealed that the ocean current facilitated the agglomeration and transportation of microplastics. [ABSTRACT FROM AUTHOR]

Marine litter has become a global issue with 'garbage patches' documented in all ocean gyres. The Pacific and Atlantic garbage patches have been well described, but there are few empirical data for the Indian Ocean. In the austral summer 2019–2020, we conducted an at-sea survey of macro-litter in the rarely investigated south-west Indian Ocean. Over 24 days, 1623 man-made items were observed including plastic fragments, packaging and fishing-related items during 216 h of observations covering 5464 km. More than 99% of the litter items were plastics of which almost 60% were white. Floating litter was patchily distributed with only five items (0.2%) recorded south of 40°S (0.1 items·km‐2). Half of the items were encountered over a two-day period south-east of Madagascar (30°S; 59–67°E; 75.2 items·km‐2). Our survey detected an accumulation of litter in the southern Indian Ocean and demonstrated that this area warrants more research. [Display omitted] • Visual survey of macro-litter in the southern Indian Ocean • A total of 1623 anthropogenic items observed over 5464 km • Almost half of items were plastic fragments <5 cm • Accumulation zone detected south-east of Madagascar • Increased pollution risk to protected megafauna and to fisheries [ABSTRACT FROM AUTHOR]